The present invention relates to storage-stable aerated gel compositions. It also relates to a process for making such compositions. In particular, the gel compositions contain a particular hydrophobic silica, water and gelling agent.
Aqueous dispersions of silica can be prepared into a state known generally in the prior art as xe2x80x9cdry waterxe2x80x9d. In fact xe2x80x9cdry waterxe2x80x9d is known in two forms. The first form can be produced by absorbing aqueous liquids onto hydrophilic material to form a material which exists as free-flowing powder or granules. The second form can be produced by coating finely divided aqueous liquids with powdered hydrophobic material, such as metal oxides. Each liquid particle in this second form of xe2x80x9cdry waterxe2x80x9d is separated from the next by a hydrophobic metal oxide coating and by air spaces. Very high speeds of, for example, over 6000 rpm, and mixing times of 15 minutes are typically required. This second form is, however, thermodynamically unstable and, when produced, tends to break down after a relatively short period of time.
The present invention is based on the discovery that stable aerated gels analogous to this second form of xe2x80x9cdry waterxe2x80x9d can be produced.
The present invention provides a storage-stable aerated gel composition comprising 30 to 97% by weight of water, 0.2 to 5% by weight of a gelling agent selected from xanthan gum, sodium alginate and neutralised carboxyvinyl polymer and 2 to 5% by weight of a fine particulate, hydrophobic silicone-treated silica having a surface area of from 80 to 300 m2/g which said composition is in the form of fine particles of an aqueous gel containing the water and gelling agent, the surfaces of which fine particles are coated with a coating of the finely particulate hydrophobic silica.
The present invention further provides a process for producing a storage-stable aerated gel composition which comprises the steps of mixing, under high shear conditions, from 2 to 5% by weight of a finely particulate, hydrophobic silicone-treated silica having a surface area of from 80 to 300 m2/g and from 30 to 97% by weight of water, adding to the resulting mixture from 0.2 to 5% by weight of a gelling agent selected from xanthan gum, sodium alginate and neutralised carboxyvinyl polymer and mixing the gelling agent with the silica-water mixture under conditions of high shear.
A discovery on which the present invention is based lies in the use of certain gelling agents which, when added to a premix formed by mixing the water and a specific type of hydrophobic silica under high shear conditions and then mixed with the premix also under high shear conditions, give a storage-stable aerated get composition. Many conventional gelling agents which are ordinarily used to gel aqueous systems do not produce storage-stable gel compositions according to the present invention. The reasons for this are not, at present, understood. Furthermore, by adjusting the components used in the present invention the structure of the gel composition produced can be varied from a free-flowing pulverulent fluid composition which may have applications using conventional spray or dusting equipment to a non-flowing composition which may have applications as a caulking material.
Where the words xe2x80x9ccomprisesxe2x80x9d or xe2x80x9ccomprisingxe2x80x9d are used herein, it is intended that these may have the meaning xe2x80x9cincludesxe2x80x9d and xe2x80x9cincludingxe2x80x9d, respectively, to the extent that the presence of one or more other materials is not excluded.
The composition of the present invention comprises a fine particulate hydrophobic silicone-treated silica having a surface area of from 80 to 300 m2/g. By the term xe2x80x9cfine particulatexe2x80x9d, as applied to the hydrophobic silica, it is meant that the hydrophobic silica will typically have an average particle size of less than 40 xcexcm. The silica used is one that has been rendered hydrophobic by surface treatment using one or more organosilicon compounds to produce, on the silicon dioxide surface, silicone groups. The technique of hydrophobicizing silica in this way is well-known and such silicone-treated silica is available commercially. We have found that good results are obtained by using hydrophobic silica marketed under the name CAB-O-SIL (xe2x80x9cCAB-O-SILxe2x80x9d is a trademark of Cabot Corporation), preferably CAB-O-SIL TS720. However, other silicone-treated silicas can also be used in the present invention if they have a surface area within the range of from 80 to 300 m2/g. The hydrophobic silica may also be one that has been surface treated to produce siloxane, as well as, silicone groups attached to the silicon dioxide surface.
The hydrophobic silica is used in an amount of from 2 to 5% by weight based on the total weight of the composition. The use of greater than 5% by weight of the hydrophobic silica results in a gel composition which is excessively dusty. The use of such a composition may give rise to a greater nuisance dust risk. Preferably, the amount of hydrophobic silica will be in the range of from 3 to 4% by weight of the composition.
The water used may typically be tap water although purified grades may be appropriate for some applications. The water will normally be used at ambient temperature since there appears to be no advantage in using heated or cooled water in the performance of the invention. The water generally will form from 30 to 97% by weight of the total composition. Preferably, however, the amount of water will be from 90 to 97% by weight to ensure the formation of aerated gel compositions of good consistency and improved stability.
As mentioned above, the gelling agent is one or more selected from xanthan gum, sodium alginate and neutralised carboxyvinyl polymers, such as carboxypolymethylene neutralised with triethanolamine. These gelling agents are included in an amount of from 0.2 to 5% by weight. The use of an amount greater than 5% by weight of the gelling agent results in a gel composition having an excessively high gel strength. Preferably, from 0.5% to 2% by weight of the gelling agent will be used depending on the desired stability and structure of the gel composition. Other hydrophilic or hydrophobic additives, known to those skilled in the art may be incorporated to modify the physical or biological properties of the composition.
The process for producing the gel compositions involves mixing the water with the silica under high shear conditions typically for a few minutes, for example 2 to 5 minutes. The mixing at this stage in the process must be carried out under high shear conditions, i.e., conditions which cause the water to be finely fragmented into minute droplets which become dispersed within the finely particulate hydrophobic silica such that the surfaces of the water droplets become coated with the hydrophobic silica particles. The term xe2x80x9chigh shearxe2x80x9d is, of course, well-known to the person skilled in the art of mixing or blending and whether or not a particular mixing apparatus is capable of mixing aqueous compositions under high shear conditions will be known to one skilled in the art. This may be achieved by using standard high speed mixers, typically using a mixing speed of at least 2000 rpm and generally from 2000 to 3000 rpm. After the hydrophobic silica and the water have been mixed to create a dispersion of fine droplets of water in the silica, the gelling agent is added and mixing at high speed is continued for several minutes until the gelling agent has been thoroughly incorporated into the liquid phase. It is preferred in the present invention to add the gelling agent after the silica and water have been thoroughly mixed together. If the gelling agent is added before the silica, mixing requires more energy and the homogeneity and stability of the resulting gel composition can be compromised.
The storage-stable aerated gel compositions of the present invention can be used in sprayable formulations produced for domestic, veterinary, agricultural or horticultural applications.
The invention will now be illustrated by the following examples in which the compositions in Examples 1 to 3 and 5 to 7 were mixed using an IKA RE166 high speed mixer having a radial flow toothed disc mixer head and the composition in Example 4 was mixed using a larger scale Torrance high speed mixer having a radial flow toothed disc mixer head.